ILED (Inorganic Light Emitting Diode) displays provide an alternative to the better known LCD (Liquid Crystal Display) and the OLED (Organic Light Emitting Diode) displays. An ILED display does not have any of the negative qualities of LCD or OLED displays as its display sub-pixels are based on ILED light sources and has all the advantages of this class of device. This results in a display that has the better performance characteristics of an OLED type direct view display as well as the robustness, long-life and stability that is inherent to ILED technology.
Display pixel failure and need for repair presents additional complexity to the display manufacturing process. The number of acceptable malfunctioning pixels in a display is covered by ISO-13460-2. Most display manufacturers supply class 2 displays which allow no more than two malfunctioning pixels (always on or always off) per million pixels. This requires a display sub-pixel yield of 99.998%.
For a 1920×1080 display (FHD displays), which are becoming increasingly common in mobile phones, there are 2,073,600 display pixels. Therefore, in an ILED type display, 6,220,800 individual ILED chips must be packaged (an R, G and B for each display pixel). As each device must have both p and n contacts this results in 12,144,600 contacts that must be made. As per the ISO-13460-2 standard only 4 display sub-pixels may be malfunctioning. Therefore the required yield for LED devices and interconnection is 99.99996%. Achieving this target is a significant challenge in the development of ILED type displays.
To increase the yield of displays, redundancy for chips/interconnection failure is used. In standard designs, this is done by placing two ILED chips at each display sub-pixel. Using this solution a total 12,144,600 individual ILED devices must be fabricated and “pick-and-placed” during assembly of the display. Subsequently, or as part of the pick-and-place process, interconnection must be made to 24,289,200 contacts. This approach can successfully reduce the issue of device failure as the likelihood of two failed devices/interconnections in the same display sub-pixel is smaller than the risk with a single device/interconnection. However, there are implications for the cost and complexity of the system. In addition, the presence of multiple structures (i.e. other chips) in close proximity to an emitting ILED may results in unwanted reflections and other optical interference which affect the performance of the display. An example of this is the contrast-reducing light reflection that may be produced at the surface of the ILED chips.